Most people who "argue" on the internet are actually making conversation. It's not structured like a debate, or logical like an argument, but it's a tussle between two personalities -- which as we know often don't resemble those found in real life.
One of the most misused phrases is "ad hominem," which is usually confused with a type of non-sequitur known as an insult.
Person A: Your point is illogical, and the rest of your message is the ramblings of a confused maniac.
Person B: That's an ad hom!
An ad hominem fallacy, as opposed to the generic term "ad hom" referring to an insult, is an argument which claims "you can't know this because of who you are."
First, an attack against the character of person making the claim, her circumstances, or her actions is made (or the character, circumstances, or actions of the person reporting the claim). Second, this attack is taken to be evidence against the claim or argument the person in question is making (or presenting).
Read that again: Second, this attack is taken to be evidence. Without this step, an "ad hom" is just an insult. Making a logical argument and ending it with and also, you're a moron! is not an ad hom; saying "your argument is invalid because you are a moron" is an ad hom.
Hope that helps all you bloviators out there in internetland.